Mechanistic insights into LSD actions at 5-HT2A-serotonin receptors LSD (lysergic acid diethylamide) -- the prototypical hallucinogen -- continues to be a frequently abused psychotomimetic agent with a life-time prevalence as high as 10.9% among all individual surveyed and as high as 3% among high school students. LSD has a complex pharmacology with significant interactions with dozens of G-protein coupled receptors (GPCRs) and it exerts primary actions at serotonin 2A (5-HT2A) receptors. Various experiments have shown that ligand binding to G protein-coupled receptors (GPCRs) can activate, inhibit, or exert no effects on the G protein-dependent signaling pathway while having similar or diverse actions on a G protein-independent pathway through ?-arrestin (?ARR). In brain, these actions can be mediated through ?ARR 1 and/or 2. In recent papers in Science and Cell, the Roth lab has reported that LSD is a potent ?ARR-biased 5-HT2A receptor agonist. In preliminary experiments with wild-type mice, we find that LSD produces hyperactivity in the open field, it disrupts prepulse inhibition, and stimulates repetitive and stereotyped responses (head-twitch, nose-pokes, retrograde walking, unsupported rearing, and grooming). In contrast, the hyperactivity is blunted and the other behaviors are abrogated in the global ?ARR2 knockout mice. Nevertheless, additional physiological and behavioral responses have been ascribed to LSD that may also involve G proteins or ?ARR1. The Overall Goal of the proposed research is to clarify the role of ?ARR in mediating the actions of LSD at 5-HT2A receptors in vitro and in vivo. Our Central Hypothesis is that ?ARR-mediated signaling through 5-HT2A receptors will play a major role in many responses to LSD. Relevance: We have already reported on the antipsychotic effects ?ARR-biased dopamine 2 receptor compounds exert on behavior. We have evidence that some of behavioral effects of LSD are mediated at least through ?ARR2. With various strains of mice we will define a role for ?ARR1 and ?ARR2 signaling through 5-HT2A receptors and, thereby, reveal how activation of this receptor may underlie hallucinations in humans.